There are some applications of Nano-Fibrillated Cellulose (NFC) that require greater purity or less long fiber than others. Conventional production of this purer product requires longer refining times to reach that condition. NFC, also commonly referred to as Microfibrillated Cellulose (MFC), Micro-Nano Fibrillated Cellulose (MNFC), Cellulose Nanomaterials, etc., are small cellulose fibers on the order of about 0.25 to 0.05 mm in length.
The knotter uses a barrier, or screen cylinder, with perforations in the 8 to 12 mm diameter range being most common, although perforations as small as 6 mm or as large as 16 have been used. The most common size is 9.5 mm diameter. Pulp stock passes through this screen cylinder, while the larger pieces of uncooked wood chips cannot pass through. Flows on the inlet side of the screen cylinder carry the knots to one end of the screen cylinder, from which they are discharged as “rejects”.
A conventional process for producing Cellulose Nanofibrils (CNF) pulp uses refiners equipped with various differing refiner plates and expends significant amounts of refining energy applied to conventional hardwood or softwood pulp. This refining process is usually a recirculation system whereby the refiner discharge is directed back into an agitated feed tank to be fed through the refiner numerous times. Energy is applied through this process to shorten the fiber, increase fibrillation, and increase CNF content of the pulp. During this process the refiner plate gaps are reduced significantly in order to maintain the desired level of energy application and CNF development. This constant application of energy and shortening of the fiber results in a decreasing ability of the refiner plates to capture the fiber on the bar edges resulting in reduced refining efficiency. This then results in potentially significant plate wear along with the additional power applied over time.
The conventional process, when run to the point of very high fines concentration such as but not limited to >85%, uses increasingly greater power as the CNF level increases. Additionally, there is always present some volume of longer fiber which can be detrimental in some applications of this product.
The conventional method of producing Cellulose Nanofibrils (CNF) is through a multistage refining or grinding process, which starts with any number of various pulp types like but not limited to bleached softwood Kraft pulp. The pulp supply can be bleached or unbleached, softwood, hardwood, virgin or recycled fiber, or other plant fiber. The refining process is carried out in a way that recirculates 42 all or a portion of the pulp supply through the refiner while applying energy to the fiber to result in an ever increasing fines level in the pulp. The process could be either continuous or batch but a typical continuous process is very equipment intensive. As the refining energy is applied, the CNF level increases but at a decreasing rate as the fiber is shortened and CNF increases. This process can be carried out in several stages; possibly each stage using a different refiner plate pattern than the previous.
As the refining process continues, the efficiency of energy transfer to the fiber decreases as the decreasing fiber length and increasing CNF level takes place. Thus, more and more energy is expended to further increase the CNF content to a desired target. A typical energy input to produce CNF at a 90% fines level is 2500-3500 kWh/ton. Some of the targeted CNF levels for certain applications may approach or exceed 95% as measured by an optical fiber length analyzer or other similar device. An alternative measurement method could be Water Retention Values with, but not limited to, targets in the 5.0 to 10.0 range. Significant amounts of energy may be expended to reach those target levels.